AVES BIRDS. 



625 



left. The remainder of the metamorphosis seems to depend 

 principally upon changes that occur in the bulbus arteriosus (b) 9 

 which is by degrees converted into the bulb of the aorta. This 

 part of the arterial system, from being a single cavity, about the 

 fifth day divides into two canals, which become gradually more and 

 more separated, and bent upon themselves. The separation of 

 the bulbus arteriosus into two vessels is, in the opinion of Pro- 

 fessor Baer, owing to the circumstance that the ventricles gradu- 

 ally become separated by a septum, which, as it becomes more 

 complete, causes two distinct currents of blood to be propelled from 

 the heart. The current coming from the right ventricle arrives 

 sooner than the other at the vascular arches, and rushes through the 

 two posterior and through the middle arch of the left side ; while 

 the gush of blood from the left ventricle fills the two anterior arches, 

 and the middle arch of the right side ; a circumstance depending 

 upon the course impressed upon the currents derived from the two 

 ventricles. Each current becomes more and more distinct ; and at 

 last each is provided with a proper channel, forming the trunks of 

 the future pulmonary artery and of the future aorta. 



It will be seen, that as yet the real aorta does not exist ; for at 



Fig. 288. 



this period of the metamorphosis all the blood passes through 

 the vascular arches that remain into the dorsal vessel ( fig. 288, m), 

 which is formed in the same manner as the aorta of Fishes by the 

 union of the branchial vessels. 



While the branchial fissures penetrated into the pharyngeal 

 cavity, the branchial vessels were contained in the corresponding 

 branchial arches ; but, as soon as these fissures disappear, the vas- 

 cular trunks abandon the neighbourhood of the pharynx, and begin 

 to assume the character that they afterwards present. 



The most posterior arch of the left side gradually disappears, 



2 s 



